The present invention relates to pilot operated proportional hydraulic valves which are electrically controlled, and particularly to calibrating the control of such valves.
The application of hydraulic fluid to an actuator, such as a cylinder and piston arrangement, can be controlled by a set of solenoid operated pilot valves. A pump supplies hydraulic fluid under pressure to an electro-hydraulic valve (EHV) assembly, such as the one described in U.S. Pat. No. 5,878,647. The EHV assembly includes a fluid distribution block on which four solenoid valves are mounted to control the flow of fluid to and from chambers of a hydraulic cylinder connected to the fluid distribution block. A first pair of the solenoid valves governs the fluid flow to and from the piston chamber of the cylinder, and a second pair of the solenoid valves controls the fluid flow to and from the rod chamber. By sending pressurized fluid into one cylinder chamber and draining fluid from the other chamber, the piston can be moved in one of two directions. The rate of flow into a chamber of the cylinder is varied by controlling the degree to which the associated supply valve is opened, which results in the piston moving at proportionally different speeds.
Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. A spring acts on the armature to close the valve when electric current is removed from the solenoid coil. An example of a solenoid operated pilot valve of this type is described in the aforementioned U.S. Patent.
Such proportional solenoid valves usually have a spring preload force that acts on the pilot poppet. As a consequence a substantial current level is required to produce an electromagnetic force that overcomes the spring force and produces opening movement of the pilot poppet. If the control circuit commences applying current to the valve from zero when the operator first moves a manual control device, that device must be moved a certain amount before sufficient current is applied to the electromagnetic coil to open the valve. This produces a dead band of wasted motion of the manual control device.
To overcome this dead band problem, control circuits have been designed to apply a predefined current level above zero upon initial movement of the control device. In other words as shown in FIG. 1, the current applied to the electromagnetic coil jumps from zero to that predefined initial current level IINT when the operator initially moves the control device from the off position. The predefined initial current level is set to produce a force on the armature of the solenoid that is slightly less than the spring preload force. Thus the valve does not open immediately when the control device is moved from the off position. As the control device continues to be moved the coil current increases causing pilot valve to open thereby producing a small flow through the valve. Eventually the coil current increases to a level IO at which the main valve poppet opens. This operation virtually eliminates the dead band of wasted operator motion. The difference between the initial current level IINT and the current level IO at which the main valve poppet opens is referred to an the xe2x80x9cmarginxe2x80x9d.
A problem in this operation arises due to relaxation of the spring preload force with age which results in the valve opening at a significantly lesser force produced by the electromagnetic coil, thus decreasing the margin. Such relaxation can result from fatigue of the valve spring, deformation of the pilot poppet-seat interface, or deformation of the main poppet-seat interface. In pressure compensated solenoid valves, changes in the compensation mechanism with age also produces relaxation of the spring preload force. When significant relaxation occurs, the valve may jump from a closed position to a substantial flow position when the initial current level is applied to the valve. This inhibits control at low flow rates.
The present invention provides a method for calibrating control of a fluid valve having an inlet, an outlet and an electrically operated actuator. When the fluid valve is to be opened, a predefined initial level of electric current is applied initially to the electrically operated actuator. The calibration involves applying pressurized fluid to the inlet of the electrically operated valve and applying an electric current at varying levels to the electrically operated actuator. The pressure at one of the inlet and the outlet is measured, thereby producing a pressure measurement which is employed to determine when the fluid valve opens. For example, opening of the valve is indicated when the rate of change of the measured pressure changes more than a given amount.
A difference between the electric current level which was being applied when the fluid valve opened and the predefined initial level then is calculated. The predefined initial level is changed in response to that difference. In the preferred embodiment of the invention, the predefined initial level is set to a fixed amount less than the level of the electric current which was being applied when the fluid valve opened. This calibration ensures that the initial level of current applied to open the valve will be a desired amount less that the current level at with the valve begins to open. Thus uniform operation of the valve occurs, even as the valve ages.